If a phone is left turned on for too long without restarting it, it can disconnect from the A2P network. It might still be able to receive messages via the P2P network but will not receive any messages via the A2P network. This is why a restart of the device usually resolves the issue, as this will force the phone to reconnect to the A2P network.

Alternatively, the phone can also be put into Flight Mode for about 20 seconds. When you turn off Flight Mode after that time, the phone usually also reconnects to the A2P network.